The subject matter disclosed herein relates to a brush seal assembly, and more specifically to a brush seal assembly having a backing plate including at least one backing tooth for selectively contacting a sealing surface.
Brush seals are commonly found in a variety of applications such as gas and steam turbines, and are typically employed to seal a gap located between a rotary shaft and a stationary member. Brush seals have a plurality of bristles that are sandwiched between a forward plate and a backing plate. The bristles extend towards a sealing surface and provide a seal between the rotary shaft and the stationary member.
The backing plate of the brush seal provides axial support to the bristles and also partially obstructs the gap located between the rotary shaft and the stationary member. A clearance is located between the backing plate and the rotary shaft in an effort to substantially prevent the backing plate from contacting the rotary shaft during operation. This clearance is sometimes referred to as the fence height of the backing plate. The fence height of the backing plate is typically more than the conventional metal-to-metal clearance. In an effort to improve brush seal pressure capability and reduce bristle wear, the fence height is reduced such that the bristles have increased support and are subjected to less bending stress from pressure loading. However, reducing fence height may increase the risk of rubbing against the rotary shaft during transient conditions, such as when the system is passing resonant speeds or in the event thermal equilibrium between components is reached. If the fence height is not adequate, the surface of the backing plate that opposes the rotary shaft may mushroom or smear when rubbed by the rotating surface of the rotary shaft during transient conditions.
In the event a backing plate tip is rubbed, the edges of the backing plate may become irregular and intermittent. This may create a difficult situation, as the bristles may not slide easily along on the backing plate. If the bristles are stuck on an irregular edge of the backing plate, the bristles may become permanently bent and worn where there are rotary shaft excursions. Thus, the backing plate should provide axial support to the bristles while still substantially avoiding contact with the rotary shaft.
Several approaches currently exist for reducing or preventing the surface of the backing plate from contacting the rotary member while still maintaining axial support of the bristles. For example, in one approach, the surface of the backing plate is coated with a tribologically compatible coating or backing layer to be more tolerant to rubbing. In another approach, a material that facilitates rubbing between the backing plate and the rotary surface could be employed as well to substantially avoid mushrooming of the backing plate tip. However, these approaches may introduce added cost and complexity to the brush seal. In another approach, the forward plate is set to a relatively tight clearance to serve as a bump element to protect the backing plate, but it is typically not a viable option, because this approach will create a jet flow that may destabilize the brush seal bristles, causing fluttering and relatively quick wear. Therefore, it would be desirable to provide a cost effective system for reducing contact between the backing plate of a brush seal and a rotary shaft.